It is well known that glass and like transparent substrates can be coated with transparent films to provide or alter optical properties, such as transmission, reflection, electrical conductivity, etc. Particularly significant commercial uses for such coatings include, for example, infrared reflection, low-emissivity and solar load reduction, etc. In solar load control applications, for example, such coatings reduce the amount of solar energy in the non-visible wavelengths passing through the glazing panel to reduce an air-conditioning load within a building, motor vehicle, etc. In a low emissivity glazing application, such coatings reduce the heating load of a building in a cold climate by reducing the loss of infrared radiation from the heated interior of the building through the glazing panel. Typically, for example, glass and other transparent materials can be coated with transparent semi-conductor films such as tin oxide, indium oxide or cadmium stannate, in order to reflect infrared radiation. Coatings of these same materials and other materials also conduct electricity, and are employed as resistance heaters to heat windows, particularly in motor vehicles, airplanes, etc. to remove fog and ice.
It is a recognized problem that substrates bearing such coatings may show iridescence, that is, color in reflected light and, to a lesser extent, in transmitted light. Such iridescence is understood to be generally the result of an interference phenomenon wherein certain wavelengths reflected partially at the exterior of the coating are out of phase with light of that wavelength reflected from the interface of the coating with the substrate, while reflected light of other wavelengths is in phase and interferes additively. The degree to which the reflected light of a given wavelength cancels or interferes additively is a function of the wavelength and the optical thickness of the coating.
The iridescence effect typically associated with coatings less than about 1 micron, especially less than about 0.75 microns, is aesthetically unacceptable in many architectural, motor vehicle and other applications. In fact, interference colors most generally occur with coatings in the thickness range of 0.1 to 1.0 micron, a thickness range of significant practical importance in many commercial applications. A large portion of present commercial production of coated glass glazing panels, for example, comprise coatings in the thickness range of about 0.1 to 1.0 micron, which display pronounced iridescent colors, especially in reflected daylight. The presence of iridescence is commonly understood to inhibit the use of more energy efficient coated glass in many glazing applications, despite the fact that the potential energy conservation would make the application cost effective. In addition, lack of thickness uniformity in the coating results in the appearance of multiple colors on a single piece of glass, sometimes referred to as banding, often rendering the glazing unit visually unacceptable.
One known means of reducing visible interference colors from such film coatings on glass or a like substrate is to increase the thickness of the coating to greater than one micron. Thicker coatings, however, are more expensive to make, requiring more reactant and longer deposition times. Furthermore, they have a greater tendency to crack as a result of thermal stress. An alternative means of reducing interference color involves the use of an underlayer coating between the substrate surface and the optically functional coating. For example, a known color suppressing undercoat for a fluorine-doped tin oxide low emissivity coating 3,000 to 4,000 Angstroms thick consists essentially of a Si-O-C interlayer between the glass substrate and the overcoat. The interlayer has a refractive index intermediate that of the substrate and the overcoat and is about 700 Angstroms thick.
In U.S. Pat. No. 4,440,822 to Gordon heat loss by infrared radiation through the glass areas of a heated building is said to be approximately one-half the heat loss through uncoated windows. The presence of iridescent colors on coated glass is said to be a major reason preventing its use. The Gordon '822 patent is directed to transparent glass window structures wherein the glass bears a coating of infrared reflective material with an interlayer of continuously varying refractive index between the glass and the coating. The refractive index of the interlayer is said to increase continuously from a low value at the interface of the interlayer with the substrate to a high value at the interface with the infrared reflective coating. FIG. 5 of that patent, for example, shows an underlayer consisting of tin oxide and silicon oxide wherein the relative proportion of tin and, hence, the refractive index, both increase with distance from the glass surface. The refractive index increases from about 1.5 at the glass surface to about 2.0 at the interface with the thick film coating of infrared reflective material. Reducing color to a low level of iridescence by interposing, between a substrate and a coating, a graded-index layer that varies in refractive index between the values at the two boundaries also is suggested in Principles of Design of Architectural Coatings, APPLIED OPTICS, Volume 22, No. 24, pp. 4127-4144 (15 Dec. 1983).
Other approaches have been suggested. In U.S. Pat. No. 4,308,316 to Gordon and in U.S. Pat. No. 4,187,336 to Gordon (a division of Gordon '316) single and double layer undercoats on glass under a thick film coating of tin oxide are taught for reducing iridescence. The one or more layers of transparent material between the glass and the semi-conductor coating are said to have refractive indices intermediate those of the glass and the semi-conductor. The double interlayer taught by these patents involves a first sub-layer closest to the glass having a lower refractive index and a second sub-layer closer to the semi-conductor coating having a relatively higher refractive index, both values being, as stated immediately above, intermediate the refractive index values of the glass and the coating.
In U.S. Pat. Nos. 4,419,386 to Gordon and 4,377,613 to Gordon (a division of Gordon '386) an intermediate layer is placed between a glass substrate and an infrared reflecting coating to reduce iridescence. The interlayer is similar to that disclosed in above mentioned U.S. Pat. No. 4,187,336 to Gordon, except that the order of refractive index is reversed. That is, the sub-layer further from the glass has the lower refractive index while the sub-layer closer to the glass has the higher refractive index. It is claimed that by reversing the order the color suppression is achieved using thinner layers.
The importance of color properties for window coatings is recognized also in Evaporated Sn-Doped In.sub.2 O.sub.3 Films: Basic Optical Properties and Applications to Energy-Efficient Windows, J.Appl.Phys. 60 (11) pp. 123-159. Section X.C of that article discusses anti-reflection treatment for significantly decreasing iridescence. It is noted that iridescence has plagued earlier oxide-type window coatings, leading manufacturers to use film thicknesses much larger than those required to obtain a desired low thermal emittance. This is noted to be inefficient in terms of materials utilization and process time. An anti-reflection coating of sputtered aluminum oxyfluoride material is mentioned.
Many such known anti-iridescence undercoats, including some of the undercoats of the Gordon patents, present a haze problem. Specifically, some of the Gordon patents admit that many of the disclosed coatings, when used on ordinary window glass, show considerable haze or scattered light. To remedy this deficiency, Gordon recommends first depositing on the glass substrate surface a layer of low refractive index material such as SiO.sub.2. Also suggested for this purpose are Si.sub.3 N.sub.4 and GeO.sub.2. In particular, it is asserted that if the initial layer contains large proportions of materials including, for example, SnO.sub.2, "then haze formation is likely."
Another difficulty connected with the anti-iridescence undercoats suggested in the Gordon patents and in other teachings is their sensitivity to the thickness of the interlayers. Specifically, the degree of anti-iridescence efficacy depends strongly on depositing the interlayers within precise thickness ranges and with highly uniform thickness. In U.S. Pat. No. 4,187,336, for example, it is suggested that a change of plus or minus 0.02 in the refractive index or a change of plus or minus 10% in the thickness of certain single layer undercoatings would be sufficient to raise the color saturation to observable values. In coated substrate production on an industrial scale, it may be difficult in certain instances to guarantee coating deposition within such narrow ranges. Certain double interlayer systems are suggested by Gordon to have broader permissible thickness variations. Coating systems with high tolerance of film thickness variations are commercially and economically desirable.
It is an object of the present invention to provide a substantially transparent glazing article having a coating with an anti-iridescence layer which is, at least in certain preferred embodiments of the invention, tolerant of deviations in its parameters, specifically, deviations in the thickness and refractive index of both the anti-iridescence interlayer and the optically functional coating (low emissivity coating, solar load control coating, etc.). In particular, it is an object of the invention to provide a substantially transparent glazing article and a method of producing the same which are robust in their industrial implementation. Specifically, it is an object of the invention to provide such glazing article wherein at least certain preferred embodiments have product and manufacturing process specifications with tolerance ranges readily achievable using presently available manufacturing techniques and equipment. These and other objects of the present invention will be better understood from the following disclosure and description thereof.